The present invention relates to a radio communication method and a base station and user terminal thereof, and more particularly to a radio communication method and a base station and user terminal thereof in a radio communication system in which each user terminal uses different data transmission band frequencies that are assigned from a base station to transmit data signals to that base station, and performs time-division multiplexing of pilot signals onto the data signal and transmits the resulting signal to the base station.
In a radio communication system such as a cellular system, the receiving side typically uses a known pilot signal to perform timing synchronization and propagation path estimation (channel estimation), and based on each of these, performs data demodulation. Moreover, in an adaptive modulation method that makes it possible to improve throughput by adaptively changing the modulation method or encoding rate according to the channel quality, the receiving side also uses the pilot signal when estimating the channel quality, for example the signal to interference ratio (SIR), in order to decide the optimal modulation method or optimal encoding rate.
As a radio communication access method that is strong against frequency selective fading due to multipaths in broadband radio communication, is the OFDM (Orthogonal Frequency Division Multiplexing) method. However, from the aspect of the power efficiency of the terminal, there is a problem that the PAPR (Peak to Average Power Ratio) of the transmission signal is large, so OFDM is not suited as a method for UP link transmission. Therefore, in the next-generation cellular system 3GPP LTE, single-carrier transmission is performed as the uplink transmission method, where the receiving side performs frequency equalization (refer to 3GPP TR25814-700 FIG. 9.1.1-1). Single-carrier transmission means that transmission data and pilot signals are multiplexed only on the time axis, and when compared with OFDM that multiplexes data and pilot signals on the frequency axis, it is possible to greatly reduce the PARR.
Single-Carrier Transmission
FIG. 23 is an example of the frame format of single-carrier transmission, and FIG. 24 is a drawing explaining frequency equalization. A frame comprises data DATA and pilots PILOT having N number of samples each and that are time multiplexed, where in FIG. 23 two pilot blocks are inserted in one frame. When performing frequency equalization, a data/pilot separation unit 1 separates data DATA and pilots PILOT, and a first FFT unit 2 performs FFT processing on N samples of data to generate N number of frequency components, and inputs the result to a channel-compensation unit 3. A second FFT unit 4 performs FFT processing on N samples of pilot to generation N number of frequency components, and a channel estimation unit 5 uses those N number of frequency components and N number of frequency components of a known pilot to estimate the channel characteristics for each frequency, and inputs a channel-compensation signal to the channel-compensation unit 3. The channel-compensation unit 3 multiplies the N number of frequency components that were output from the first FFT unit 2 by the channel-compensation signal for each frequency to perform channel compensation, and an IFFT unit 6 performs IFFT processing of the N number of channel-compensated frequency components, then converts the signal to a time signal and outputs the result.
CAZAC Sequence
In single-carrier transmission, when the receiving side performs frequency equalization, in order to improve the accuracy of channel estimation in the frequency domain, it is preferred that the pilot signal has a constant amplitude in the frequency domain, or in other words, that the auto correlation after an arbitrary cyclic time shift be ‘0’. On the other hand, from the aspect of the PAPR, it is preferred that a pilot signal has a constant amplitude in the time domain as well. A pilot sequence that makes these features possible is a CAZAC (Constant Amplitude Zero Auto Correlation) sequence, and in the 3GPP LTE system, application of this CAZAC sequence as the up link pilot is decided. The CAZAC sequence has ideal auto correlation characteristics, so pilot signals that are obtained by cyclically shifting the same CAZAC sequence are orthogonal to each other. In the 3GPP LTE system, a method of using CAZAC sequences having different amounts of cyclic shift to multiplex the pilot signals of different users, or to multiplex pilot signals from the same user but transmitted from different antennas is adopted and it is called CDM (Code Division Multiplexing).
A Zadoff-Chu sequence, which is a typical CAZAC sequence, is expressed by Equation (1) (refer to B. M. Popovic, “Generalized Chirp-Like Polyphase Sequences with Optimum Correlation Properties”, IEEE Trans. Info. Theory, Vol. 38, pp. 1406-14 09, July 1992).ZCk(n)=exp{−j2πk/L·(qn+n(n+L %2)/2)}  (1)Here, k and L are both prime, and express the sequence number and sequence length, respectively. Moreover, n is the symbol number, q is an arbitrary integer, and L %2 is the remainder when divided by 2, and may be notated as Lmod(2). When the factorization into prime numbers of L is taken to beL=g1e1× . . . ×gnen  (2)(gi is a prime number), the number of CAZAC sequences can be given by the following equation.
                              ϕ          ⁡                      (            L            )                          =                              L            ⁡                          (                              1                -                                  1                                      g                    1                                                              )                                ×          …          ×                      (                          1              -                              1                                  g                  n                                                      )                                              (        3        )            More specifically, in the case where L=12, L=12=22×31, so g1=2, e1=2, g2=3 and e2=1, and from Equation (3), the number of sequences (CAZAC sequences) becomes 4. Therefore, the number of sequences increases the larger L is and the fewer number of prime factors there is. In other words, in the case where L is a prime number, the number of CAZAC sequences φ(L) becomes (L−1).
ZCk(n−c), for which only c in the CAZAC sequence ZCk(n) is cyclically shifted, is expressed by the following equation.ZCk(n−c)=exp{−j2πk/L·(q(n−c)+(n−c)(n−c+L %2)/2)}  (4)As is shown in Equation (5) below,
                                                    R            ⁡                          (              τ              )                                                =                  {                                                                                          1                    ⁢                                                                                  ⁢                    …                    ⁢                                                                                  ⁢                    τ                                    =                  c                                                                                                                          0                    ⁢                                                                                  ⁢                    …                    ⁢                                                                                  ⁢                    τ                                    ≠                  c                                                                                        (        5        )            the correlation R(τ) between ZCk(n) and ZCk(n−c) becomes ‘0’ at any point except where τ=c, so sequences that are obtained by applying different amounts of cyclic shift to the main sequence ZCk(n) become orthogonal to each other.
When a radio base station receives a plurality of pilots that were multiplexed by CDM (Code Division Multiplex) using the cyclic shift, by taking the correlation with the main sequence, it is possible to separate the pilots based on the location where the peak occurs. The ability to tolerate shifting of the multipath or shifting of the reception timing decreases the narrower the interval of the cyclic shift is, so there is an upper limit to the number of pilots that can be multiplexed by cyclic shift. When the number of pilots that are multiplexed by cyclic shifting is taken to be P, the amount of cyclic shifting cp that is assigned to the pth pilot can be determined, for example, by the equation given below (refer to 3GPP R1-060374, “Text Proposal On Uplink Reference Signal Structure”, Texas Instruments).cp=(p−1)*{L/P], where p=1, . . . , P  (6)
As was described above, in a 3GPP LTE uplink, pilots and data are multiplexed by time-division multiplexing and transmitted by the SC-FDMA (Single Carrier-Frequency Division Multiple Access) method. FIG. 25 is a drawing showing the construction of a SC-FDMA transmission unit, where 7′ is a NTX sized DFT (Discrete Fourier Transformer), 8′ is a subcarrier mapping unit, 9′ is a NFFT sized IDFT unit, and 10 is CP (Cyclic Prefix) insertion unit. In 3GPP LTE, in order to suppress the amount of processing, NFFT is an integer that is a power of 2, and the IDFT after subcarrier mapping is replaced by IFFT.
The process of adding a cyclic shift c to the main sequence ZCk(n) can be performed either before DFT or after IFFT. When the process is performed after IFFT, the cyclic shift can be an amount c×NFFT/NTX samples. Essentially, the process is the same process, so hereafter, an example will be explained in which the cyclic shift process is performed before DFT.
Problems with the Related Art
In order to reduce inter-cell interference, it is necessary to repeatedly use CAZAC sequences having different sequence numbers as pilots between cells. This is because as the number of repetitions increases, the distance between cells that use the same sequence becomes larger, so the possibility of severe interference occurring decreases. Therefore, it becomes necessary to maintain a lot of CAZAC sequences, and in order to have good characteristics for the CAZAC sequences, a sequence length L that is a large prime number is desirable. FIG. 26 is a drawing explaining inter-cell interference, where in the case as shown in (A), in which the number of CAZAC sequences that can be used is 2, CAZAC sequences (ZC1) having the same sequence number are used in adjacent cells, so severe interference occurs between the adjacent cells. Moreover, as shown in (B), when the number of CAZAC sequences is 3, CAZAC sequences having the same sequence number are not used, however, the number of repetitions is 3, which is a small number, so the distance between cells that use CAZAC sequences having the same sequence number is short and there is a high possibility that interference will occur between adjacent cells. In the case shown in (C), where the number of CAZAC sequences is 7, the number of repetitions is 7, which is a large number, so as the distance between cells that use CAZAC sequences having the same sequence number becomes larger, the possibility of interference occurring gradually decreases.
Incidentally, as shown in (A) of FIG. 27, the trend of 3GPP LTE discussion is to take the number of subcarriers that are occupied by data a multiple of 12, and to take the subcarrier interval for pilots double the subcarrier interval for data in order to improve the transmission efficiency. In that case, when the sequence length L of the CAZAC sequence is 6, the number of sequences φ(L) becomes 2(k=1, 2), and CAZAC sequences having the same sequence number are used, so pilot interference occurs between adjacent cells. Moreover, when the sequence length L is taken to be 5, φ(L) becomes 4(k=1, 2, 3, 4), which is still a small number, however, as shown in (B) of FIG. 27, there are subcarriers of data that are not covered by a pilot, so the channel estimation accuracy decreases.
Therefore, it is thought that by making the transmission band for pilot signals broader than the transmission band for data and performing transmission, a sufficient sequence length L will be maintained (refer to 3GPP R1-060925, R1-063183). FIG. 28 is an example of the case in which the number of multiplexed pilot signals is 2. If the sequence length L is taken to be 12, the number of CAZAC sequences is only 4 from the equations (2) and (3), and the inter-cell interference becomes large (k=4). Therefore, the sequence length L is made to be the prime number 11. When L=11, φ(L) is 10 and 10 CAZAC sequences can be used (k=1˜10), so it is possible to reduce the inter-cell interference. The sequence length L cannot be made to be 13 or greater. The reason for that is that when the sequence length L is 13 or greater, interference occurs between adjacent frequency bands.
Pilot signals from different users are multiplexed by CDM through cyclic shifting. In other words, a CAZAC sequence ZCk(n) having a length L=11 and for which cyclic shifting c1 has been performed is used as the pilot for a user 1, and a CAZAC sequence ZCk(n) for which cyclic shifting c2 has been performed is used as the pilot for a user 2.
However, when a CAZAC sequence ZCk(n) having a length L=11 is cyclically shifted and used for the users 1, 2, then as can be clearly seen in FIG. 28, the relative relationship between the transmission frequency band for the pilots and the transmission frequency band for data for user 1 and user 2 differs, and thus the channel estimation accuracy is different. In other words, subcarriers 23, 24 of the transmission frequency band for data of user 2 deviates from the transmission frequency band for the pilots, and the channel estimation accuracy for those subcarriers decreases.
In FIG. 28, based on the current 3GPP LTE specifications, the subcarrier interval for pilots is double the subcarrier interval for data, however, the problem described above occurs even when the ratio of the subcarrier intervals is changed.